Knock-in Mouse Line Research Articles

Physiological Expression of AMPKγ2RG Mutation Causes Wolff-Parkinson-White Syndrome and Induces Kidney Injury in Mice

Mutations of the AMP-activated kinase gamma 2 subunit (AMPKγ2), N488I (AMPKγ2NI) and R531G (AMPKγ2RG), are associated with Wolff-Parkinson-White (WPW) syndrome, a cardiac disorder characterized by ventricular pre-excitation in humans. Cardiac-specific transgenic overexpression of human AMPKγ2NI or AMPKγ2RG leads to constitutive AMPK activation and the WPW phenotype in mice. However, overexpression of these mutant proteins also caused profound, non-physiological increase in cardiac glycogen, which might abnormally alter the true phenotype. To investigate whether physiological levels of AMPKγ2NI or AMPKγ2RG mutation cause WPW syndrome and metabolic changes in other organs, we generated two knock-in mouse lines on the C57BL/6N background harboring mutations of human AMPKγ2NI and AMPKγ2RG, respectively. Similar to the reported phenotypes of mice overexpressing AMPKγ2NI or AMPKγ2RG in the heart, both lines developed WPW syndrome and cardiac hypertrophy; however, these effects were independent of cardiac glycogen accumulation. Compared with AMPKγ2WT mice, AMPKγ2NI and AMPKγ2RG mice exhibited reduced body weight, fat mass, and liver steatosis when fed with a high fat diet (HFD). Surprisingly, AMPKγ2RG but not AMPKγ2NI mice fed with an HFD exhibited severe kidney injury characterized by glycogen accumulation, inflammation, apoptosis, cyst formation, and impaired renal function. These results demonstrate that expression of AMPKγ2NI and AMPKγ2RG mutations at physiological levels can induce beneficial metabolic effects but that this is accompanied by WPW syndrome. Our data also reveal an unexpected effect of AMPKγ2RG in the kidney, linking lifelong constitutive activation of AMPK to a potential risk for kidney dysfunction in the context of an HFD.

Single and Transient Ca2+ Peaks in Podocytes do not induce Changes in Glomerular Filtration and Perfusion

Chronic alterations in calcium (Ca2+) signalling in podocytes have been shown to cause proteinuria and progressive glomerular diseases. However, it is unclear whether short Ca2+ peaks influence glomerular biology and cause podocyte injury. Here we generated a DREADD (Designer Receptor Exclusively Activated by a Designer Drug) knock-in mouse line to manipulate intracellular Ca2+ levels. By mating to a podocyte-specific Cre driver we are able to investigate the impact of Ca2+ peaks on podocyte biology in living animals. Activation of the engineered G-protein coupled receptor with the synthetic compound clozapine-N-oxide (CNO) evoked a short and transient Ca2+ peak in podocytes immediately after CNO administration in vivo. Interestingly, this Ca2+ peak did neither affect glomerular perfusion nor filtration in the animals. Moreover, no obvious alterations in the glomerular morphology could be observed. Taken together, these in vivo findings suggest that chronic alterations and calcium overload rather than an induction of transient Ca2+ peaks contribute to podocyte disease.

Exome sequencing in the knockin mice generated using the CRISPR/Cas system.

Knockin (KI) mouse carrying a point mutation has been an invaluable tool for disease modeling and analysis. Genome editing technologies using the CRISPR/Cas system has emerged as an alternative way to create KI mice. However, if the mice carry nucleotide insertions and/or deletions (InDels) in other genes, which could have unintentionally occurred during the establishment of the KI mouse line and potentially have larger impact than a point mutation, it would confound phenotyping of the KI mice. In this study, we performed whole exome sequencing of multiple lines of F1 heterozygous Ntrk1 KI mice generated using the CRISPR/Cas system in comparison to that of a wild-type mouse used as a control. We found three InDels in four KI mice but not in a control mouse. In vitro digestion assay suggested that each InDel occurred as a de novo mutation, was carried-over from the parental mice, or was incorporated through the Cas9 nuclease mediated off-target cleavage. These results suggest that frequency of InDels found in KI mice generated by the CRISPR/Cas technology is not high, but cannot be neglected and careful assessment of these mutations is warranted.

Class II major histocompatibility complex mutant mice to study the germ-line bias of T-cell antigen receptors

The interaction of αβ T-cell antigen receptors (TCRs) with peptides bound to MHC molecules lies at the center of adaptive immunity. Whether TCRs have evolved to react with MHC or, instead, processes in the thymus involving coreceptors and other molecules select MHC-specific TCRs de novo from a random repertoire is a longstanding immunological question. Here, using nuclease-targeted mutagenesis, we address this question in vivo by generating three independent lines of knockin mice with single-amino acid mutations of conserved class II MHC amino acids that often are involved in interactions with the germ-line-encoded portions of TCRs. Although the TCR repertoire generated in these mutants is similar in size and diversity to that in WT mice, the evolutionary bias of TCRs for MHC is suggested by a shift and preferential use of some TCR subfamilies over others in mice expressing the mutant class II MHCs. Furthermore, T cells educated on these mutant MHC molecules are alloreactive to each other and to WT cells, and vice versa, suggesting strong functional differences among these repertoires. Taken together, these results highlight both the flexibility of thymic selection and the evolutionary bias of TCRs for MHC.

Pou4f2-GFP knock-in mouse line: A model for studying retinal ganglion cell development.

Pou4f2 acts as a key node in the comprehensive and step-wise gene regulatory network (GRN) and regulates the development of retinal ganglion cells (RGCs). Accordingly, deletion of Pou4f2 results in RGC axon defects and apoptosis. To investigate the GRN involved in RGC regeneration, we generated a mouse line with a POU4F2-green fluorescent protein (GFP) fusion protein expressed in RGCs. Co-localization of POU4F2 and GFP in the retina and brain of Pou4f2-GFP/+ heterozygote mice was confirmed using immunofluorescence analysis. Compared with those in wild-type mice, the expression patterns of POU4F2 and POU4F1 and the co-expression patterns of ISL1 and POU4F2 were unaffected in Pou4f2-GFP/GFP homozygote mice. Moreover, the quantification of RGCs showed no significant difference between Pou4f2-GFP/GFP homozygote and wild-type mice. These results demonstrated that the development of RGCs in Pou4f2-GFP/GFP homozygote mice was the same as in wild-type mice. Thus, the present Pou4f2-GFP knock-in mouse line is a useful tool for further studies on the differentiation and regeneration of RGCs.

LGR4 and LGR5 Regulate Hair Cell Differentiation in the Sensory Epithelium of the Developing Mouse Cochlea

In the developing cochlea, Wnt/β-catenin signaling positively regulates the proliferation of precursors and promotes the formation of hair cells by up-regulating Atoh1 expression. Not much, however, is known about the regulation of Wnt/β-catenin activity in the cochlea. In multiple tissues, the activity of Wnt/β-catenin signaling is modulated by an interaction between LGR receptors and their ligands from the R-spondin family. The deficiency in Lgr4 and Lgr5 genes leads to developmental malformations and lethality. Using the Lgr5 knock-in mouse line we show that loss of LGR5 function increases Wnt/β-catenin activity in the embryonic cochlea, resulting in a mild overproduction of inner and outer hair cells (OHC). Supernumerary hair cells are likely formed due to an up-regulation of the “pro-hair cell” transcription factors Atoh1, Nhlh1, and Pou4f3. Using a hypomorphic Lgr4 mouse model we showed a mild overproduction of OHCs in the heterozygous and homozygous Lgr4 mice. The loss of LGR4 function prolonged the proliferation in the mid-basal turn of E13 cochleae, causing an increase in the number of SOX2-positive precursor cells within the pro-sensory domain. The premature differentiation of hair cells progressed in a medial to lateral gradient in Lgr4 deficient embryos. No significant up-regulation of Atoh1 was observed following Lgr4 deletion. Altogether, our findings suggest that LGR4 and LGR5 play an important role in the regulation of hair cell differentiation in the embryonic cochlea.

LGR4 and LGR5 Regulate Hair Cell Differentiation in the Sensory Epithelium of the Developing Mouse Cochlea: Lgr4/5 in the developing cochlea

In the developing cochlea, Wnt/β-catenin signaling positively regulates the proliferationof precursors and promotes the formation of hair cells by up-regulating Atoh1expression. Not much, however, is known about the regulation of Wnt/β-catenin activityin the cochlea. In multiple tissues, the activity of Wnt/β-catenin signaling is modulated byan interaction between LGR receptors and their ligands from the R-spondin family. Thedeficiency in Lgr4 and Lgr5 genes leads to developmental malformations and lethality.Using the Lgr5 knock-in mouse line we show that loss of LGR5 function increasesWnt/β-catenin activity in the embryonic cochlea, resulting in a mild overproduction ofinner and outer hair cells (OHC). Supernumerary hair cells are likely formed due toan up-regulation of the “pro-hair cell” transcription factors Atoh1, Nhlh1, and Pou4f3.Using a hypomorphic Lgr4 mouse model we showed a mild overproduction of OHCs inthe heterozygous and homozygous Lgr4 mice. The loss of LGR4 function prolongedthe proliferation in the mid-basal turn of E13 cochleae, causing an increase in thenumber of SOX2-positive precursor cells within the pro-sensory domain. The prematuredifferentiation of hair cells progressed in a medial to lateral gradient in Lgr4 deficientembryos. No significant up-regulation of Atoh1 was observed following Lgr4 deletion.Altogether, our findings suggest that LGR4 and LGR5 play an important role in theregulation of hair cell differentiation in the embryonic cochlea.

Generation and characterization of tamoxifen-inducible Pax9-CreER knock-in mice using CrispR/Cas9.

Pax9 encodes a paired-box homeodomain (Pax) transcription factor and is critical for the development of multiple organs. Using CrispR/Cas9-mediated homologous directed repair (HDR), we generated a new Pax9-CreER knock-in mouse line in which the CreER(T2) fusion protein is produced after synthesis of endogenous Pax9 protein. We found that tdTomato reporter expression in Pax9-CreER;tdTomato reporter mice is detectable in a similar pattern to the endogenous Pax9 expression, faithfully recapitulating the Pax9 expression domains throughout the embryo and in the adult mouse. At early embryonic stages, the tdTomato reporter is expressed first in the pharyngeal pouch region and later in the craniofacial mesenchyme, somites, limbs, and lingual papillae in the adult tongue. These results demonstrate that this new Pax9-CreER knock-in mouse line can be used for lineage tracing and genetic targeting of Pax9-expressing cells and their progeny in a temporally and spatially controlled manner during development and organogenesis.

Generation of Two Noradrenergic-Specific Dopamine-Beta-Hydroxylase-FLPo Knock-In Mice Using CRISPR/Cas9-Mediated Targeting in Embryonic Stem Cells.

CRISPR/Cas9 mediated DNA double strand cutting is emerging as a powerful approach to increase rates of homologous recombination of large targeting vectors, but the optimization of parameters, equipment and expertise required remain barriers to successful mouse generation by single-step zygote injection. Here, we sought to apply CRISPR/Cas9 methods to traditional embryonic stem (ES) cell targeting followed by blastocyst injection to overcome the common issues of difficult vector construction and low targeting efficiency. To facilitate the study of noradrenergic function, which is implicated in myriad behavioral and physiological processes, we generated two different mouse lines that express FLPo recombinase under control of the noradrenergic-specific Dopamine-Beta-Hydroxylase (DBH) gene. We found that by co-electroporating a circular vector expressing Cas9 and a locus-specific sgRNA, we could target FLPo to the DBH locus in ES cells with shortened 1 kb homology arms. Two different sites in the DBH gene were targeted; the translational start codon with 6–8% targeting efficiency, and the translational stop codon with 75% targeting efficiency. Using this approach, we established two mouse lines with DBH-specific expression of FLPo in brainstem catecholaminergic populations that are publically available on MMRRC (MMRRC_041575-UCD and MMRRC_041577-UCD). Altogether, this study supports simplified, high-efficiency Cas9/CRISPR-mediated targeting in embryonic stem cells for production of knock-in mouse lines in a wider variety of contexts than zygote injection alone.

Activation of the ATF6 branch of the unfolded protein response in neurons improves stroke outcome

Impaired function of the endoplasmic reticulum (ER stress) is a hallmark of many human diseases including stroke. To restore ER function in stressed cells, the unfolded protein response (UPR) is induced, which activates 3 ER stress sensor proteins including activating transcription factor 6 (ATF6). ATF6 is then cleaved by proteases to form the short-form ATF6 (sATF6), a transcription factor. To determine the extent to which activation of the ATF6 UPR branch defines the fate and function of neurons after stroke, we generated a conditional and tamoxifen-inducible sATF6 knock-in mouse. To express sATF6 in forebrain neurons, we crossed our sATF6 knock-in mouse line with Emx1-Cre mice to generate ATF6-KI mice. After the ATF6 branch was activated in ATF6-KI mice with tamoxifen, mice were subjected to transient middle cerebral artery occlusion. Forced activation of the ATF6 UPR branch reduced infarct volume and improved functional outcome at 24 h after stroke. Increased autophagic activity at early reperfusion time after stroke may contribute to the ATF6-mediated neuroprotection. We concluded that the ATF6 UPR branch is crucial to ischemic stroke outcome. Therefore, boosting UPR pro-survival pathways may be a promising therapeutic strategy for stroke.

A Phenotyping Regimen for Genetically Modified Mice Used to Study Genes Implicated in Human Diseases of Aging.

Age-related diseases are becoming increasingly prevalent and the burden continues to grow as our population ages. Effective treatments are necessary to lessen the impact of debilitating conditions but remain elusive in many cases. Only by understanding the causes and pathology of diseases associated with aging, can scientists begin to identify potential therapeutic targets and develop strategies for intervention. The most common age-related conditions are neurodegenerative disorders such as Parkinson's disease and blindness. Age-related macular degeneration (AMD) is the leading cause of blindness in the elderly. Genome wide association studies have previously identified loci that are associated with increased susceptibility to this disease and identified two regions of interest: complement factor H (CFH) and the 10q26 locus, where the age-related maculopathy susceptibility 2 (ARMS2) and high-temperature requirement factor A1 (HtrA1) genes are located. CFH acts as a negative regulator of the alternative pathway (AP) of the complement system while HtrA1 is an extracellular serine protease. ARMS2 is located upstream of HtrA1 in the primate genome, although the gene is absent in mice. To study the effects of these genes, humanized knock-in mouse lines of Cfh and ARMS2, knockouts of Cfh, HtrA1, HtrA2, HtrA3 and HtrA4 as well as a conditional neural deletion of HtrA2 were generated. Of all the genetically engineered mice produced only mice lacking HtrA2, either systemically or in neural tissues, displayed clear phenotypes. In order to examine these mice thoroughly and systematically, an initial phenotyping schedule was established, consisting of a series of tests related to two main diseases of interest: AMD and Parkinson's. Genetically modified mice can be subjected to appropriate experiments to identify phenotypes that may be related to the associated diseases in humans. A phenotyping regimen with a mitochondrial focus is presented here alongside representative results from the tests of interest.

Establishment of Nephrin Reporter Mice and Use for Chemical Screening.

Nephrin is a critical component of glomerular filtration barrier, which is important to maintain glomerular structure and avoid proteinuria. Downregulation of nephrin expression is commonly observed at early stage of glomerular disorders, suggesting that methods to increase nephrin expression in podocytes may have therapeutic utility. Here, we generated a knockin mouse line carrying single copy of 5.5 kb nephrin promoter controlling expression of enhanced green fluorescent protein (EGFP) at Rosa26 genomic locus (Nephrin-EGFP mouse). In these mice, EGFP was specifically expressed in podocytes. Next, we isolated and cultivated glomeruli from these mice, and developed a protocol to automatically quantitate EGFP expression in cultured glomeruli. EGFP signal was markedly reduced after 5 days of culture but reduction was inhibited by vitamin D treatment. We confirmed that vitamin D increased mRNA and protein expression of endogenous nephrin in cultivated glomeruli. Thus, we generated a mouse line converting nephrin promoter activity into fluorescence, which can be used to screen compounds having activity to enhance nephrin gene expression.

Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis

Microtubules play a crucial role in the generation, migration and differentiation of nascent neurons in the developing vertebrate brain. Mutations in the constituents of microtubules, the tubulins, are known to cause an array of neurological disorders, including lissencephaly, polymicrogyria and microcephaly. In this study we explore the genetic and cellular mechanisms that cause TUBB5-associated microcephaly by exploiting two new mouse models: a conditional E401K knock-in, and a conditional knockout animal. These mice present with profound microcephaly due to a loss of upper-layer neurons that correlates with massive apoptosis and upregulation of p53. This phenotype is associated with a delay in cell cycle progression and ectopic DNA elements in progenitors, which is dependent on the dosage of functional Tubb5. Strikingly, we report ectopic Sox2-positive progenitors and defects in spindle orientation in our knock-in mouse line, which are absent in knockout animals. This work sheds light on the functional repertoire of Tubb5, reveals that the E401K mutation acts by a complex mechanism, and demonstrates that the cellular pathology driving TUBB5-associated microcephaly is cell death.

Palovarotene Inhibits Heterotopic Ossification and Maintains Limb Mobility and Growth in Mice With the Human ACVR1(R206H) Fibrodysplasia Ossificans Progressiva (FOP) Mutation.

Fibrodysplasia ossificans progressiva (FOP), a rare and as yet untreatable genetic disorder of progressive extraskeletal ossification, is the most disabling form of heterotopic ossification (HO) in humans and causes skeletal deformities, movement impairment, and premature death. Most FOP patients carry an activating mutation in a bone morphogenetic protein (BMP) type I receptor gene, ACVR1(R206H) , that promotes ectopic chondrogenesis and osteogenesis and, in turn, HO. We showed previously that the retinoic acid receptor γ (RARγ) agonist palovarotene effectively inhibited HO in injury-induced and genetic mouse models of the disease. Here we report that the drug additionally prevents spontaneous HO, using a novel conditional-on knock-in mouse line carrying the human ACVR1(R206H) mutation for classic FOP. In addition, palovarotene restored long bone growth, maintained growth plate function, and protected growing mutant neonates when given to lactating mothers. Importantly, palovarotene maintained joint, limb, and body motion, providing clear evidence for its encompassing therapeutic potential as a treatment for FOP. © 2016 American Society for Bone and Mineral Research.

Generation and analysis of an improved Foxg1-IRES-Cre driver mouse line

Foxg1 expression is highly restricted to the telencephalon and other head structures in the early embryo. This expression pattern has been exploited to generate conditional knockout mice, based on a widely used Foxg1-Cre knock-in line (Foxg1tm1(cre)Skm), in which the Foxg1 coding region was replaced by the Cre gene. The utility of this line, however, is severely hampered for two reasons: (1) Foxg1-Cre mice display ectopic and unpredictable Cre activity, and (2) Foxg1 haploinsufficiency can produce neurodevelopmental phenotypes. To overcome these issues, we have generated a new Foxg1-IRES-Cre knock-in mouse line, in which an IRES-Cre cassette was inserted in the 3′UTR of Foxg1 locus, thus preserving the endogenous Foxg1 coding region and un-translated gene regulatory sequences in the 3′UTR, including recently discovered microRNA target sites. We further demonstrate that the new Foxg1-IRES-Cre line displays consistent Cre activity patterns that recapitulated the endogenous Foxg1 expression at embryonic and postnatal stages without causing defects in cortical development. We conclude that the new Foxg1-IRES-Cre mouse line is a unique and advanced tool for studying genes involved in the development of the telencephalon and other Foxg1-expressing regions starting from early embryonic stages.

Mutations in the murine homologue of TUBB5 cause microcephaly by perturbing cell cycle progression and inducing p53-associated apoptosis.

Microtubules play a crucial role in the generation, migration and differentiation of nascent neurons in the developing vertebrate brain. Mutations in the constituents of microtubules, the tubulins, are known to cause an array of neurological disorders, including lissencephaly, polymicrogyria and microcephaly. In this study we explore the genetic and cellular mechanisms that cause TUBB5-associated microcephaly by exploiting two new mouse models: a conditional E401K knock-in, and a conditional knockout animal. These mice present with profound microcephaly due to a loss of upper-layer neurons that correlates with massive apoptosis and upregulation of p53. This phenotype is associated with a delay in cell cycle progression and ectopic DNA elements in progenitors, which is dependent on the dosage of functional Tubb5. Strikingly, we report ectopic Sox2-positive progenitors and defects in spindle orientation in our knock-in mouse line, which are absent in knockout animals. This work sheds light on the functional repertoire of Tubb5, reveals that the E401K mutation acts by a complex mechanism, and demonstrates that the cellular pathology driving TUBB5-associated microcephaly is cell death.

Characterizations of Three Major Cysteine Sensors of Keap1 in Stress Response.

The Keap1-Nrf2 system plays a central role in cytoprotection against electrophilic/oxidative stresses. Although Cys151, Cys273, and Cys288 of Keap1 are major sensor cysteine residues for detecting these stresses, it has not been technically feasible to evaluate the functionality of Cys273 or Cys288, since Keap1 mutants that harbor substitutions in these residues and maintain the ability to repress Nrf2 accumulation do not exist. To overcome this problem, we systematically introduced amino acid substitutions into Cys273/Cys288 and finally identified Cys273Trp and Cys288Glu mutations that do not affect Keap1's ability to repress Nrf2 accumulation. Utilizing these Keap1 mutants, we generated stable murine embryonic fibroblast (MEF) cell lines and knock-in mouse lines. Our analyses with the MEFs and peritoneal macrophages from the knock-in mice revealed that three major cysteine residues, Cys151, Cys273, and Cys288, individually and/or redundantly act as sensors. Based on the functional necessity of these three cysteine residues, we categorized chemical inducers of Nrf2 into four classes. Class I and II utilizes Cys151 and Cys288, respectively, while class III requires all three residues (Cys151/Cys273/Cys288), while class IV inducers function independently of all three of these cysteine residues. This study thus demonstrates that Keap1 utilizes multiple cysteine residues specifically and/or collaboratively as sensors for the detection of a wide range of environmental stresses.

Mislocalization of neuronal tau in the absence of tangle pathology in phosphomutant tau knockin mice

Hyperphosphorylation and fibrillar aggregation of the microtubule-associated protein tau are key features of Alzheimer's disease and other tauopathies. To investigate the involvement of tau phosphorylation in the pathological process, we generated a pair of complementary phosphomutant tau knockin mouse lines. One exclusively expresses phosphomimetic tau with 18 glutamate substitutions at serine and/or threonine residues in the proline-rich and first microtubule-binding domains to model hyperphosphorylation, whereas its phosphodefective counterpart has matched alanine substitutions. Consistent with expected effects of genuine phosphorylation, association of the phosphomimetic tau with microtubules and neuronal membranes is severely disrupted in vivo, whereas the phosphodefective mutations have more limited or no effect. Surprisingly, however, age-related mislocalization of tau is evident in both lines, although redistribution appears more widespread and more pronounced in the phosphomimetic tau knockin. Despite these changes, we found no biochemical or immunohistological evidence of pathological tau aggregation in mice of either line up to at least 2 years of age. These findings raise important questions about the role of tau phosphorylation in driving pathology in human tauopathies.

Ezh2 and Runx1 Mutations Targeted to Early Lymphoid Progenitors Collaborate to Promote Early Thymic Progenitor Leukemia

Understanding the specific cell populations responsible for propagation of leukemia is an important step for development of effective targeted therapies. Recently, the lymphoid-primed multipotent progenitor (LMPP) has been proposed to be a key propagating population in acute myeloid leukemia (AML; PMID 21251617). We have also shown that LMPPs share many functional and gene expression properties with early thymic progenitors (ETPs; PMID 22344248). This finding is of particular interest as ETP leukemias have recently been described: a distinct and poor prognostic disease entity with a transcriptional profile reminiscent of murine ETPs, showing co-expression of hematopoietic stem cell (HSC) and myeloid markers (PMID 19147408). Together, this raises the question whether ETPs can act as a leukemia-initiating/propagating cell population; however, relevant disease models to test this hypothesis are currently lacking.Analysis of the genetic landscape of ETP leukemias has revealed frequent coexistence of inactivating mutations of EZH2 and RUNX1 (PMID 22237106). We therefore generated mice with deletions of Ezh2 and Runx1 specifically targeted to early lymphoid progenitors using Rag1Cre (Ezh2fl/flRunx1fl/flRag1Cre+; DKO mice). As anticipated, HSCs lacked significant recombination in DKO mice whereas close to 100% of purified ETPs (Lin- CD4- CD8- CD44+ CD25- Kit+ Flt3+) showed deletion of Ezh2 and Runx1. Strikingly, despite a 16-fold reduction in thymus cellularity caused by a block in thymocyte maturation at the DN2-DN3 transition, absolute numbers of ETPs within the thymus of DKO mice were markedly expanded (12-fold; p<0.0001). In contrast, Ezh2 or Runx1 deletion alone had no impact on numbers of ETPs. RNA-sequencing of the expanded ETPs in DKO mice revealed upregulation of HSC- and myeloid-associated transcriptional programs, reminiscent of ETP leukaemia e.g. Pbx1 (log2FC=3.0; p<0.0001) and Csf3r (log2FC=1.9; p=0.0038). Single-cell gene expression analysis confirmed co-expression of HSC and myeloid programs with lymphoid genes within individual DKO ETPs. Further, some key regulators of T-cell maturation which are aberrantly expressed in ETP leukemia were also disrupted in DKO ETPs e.g. Tcf7 (log2FC=-9.5; p<0.0001). Gene expression associated with aberrant Ras signalling was also present. However, despite a continued expansion of the ETP population with age, we did not observe leukemia in DKO mice with over 1 year of follow-up.Since ETP leukemias frequently feature activating mutations in genes regulating RAS signaling, we hypothesised that the expanded "pre-leukemic" ETPs in DKO mice would be primed for leukemic transformation by signalling pathway mutation. We therefore crossed DKO mice with a Flt3ITD/+ knock-in mouse line, as internal tandem duplications (ITD) of FLT3 are frequent in ETP leukemias. Ezh2fl/flRunx1fl/flRag1Cre+Flt3ITD/+ (DKOITD) mice showed dramatically reduced survival (median 9.3 weeks) resulting from an aggressive, fully penetrant acute leukemia showing a predominantly myeloid phenotype (e.g. Mac1) but with co-expression of some lymphoid antigens (e.g. intracellular CD3). Crucially, this leukaemia could be propagated in wild-type recipients upon transplantation of the expanded ETPs. DKOITD ETPs were transcriptionally very similar to DKO ETPs, retaining expression of lymphoid alongside HSC- and myeloid-associated genes.Finally, in a lympho-myeloid cell line model (EML cells) we demonstrated that Ezh2 inactivation-induced loss of H3K27me3 is associated with a corresponding increase in H3K27Ac, a transcriptional activating signal that recruits bromodomain proteins. As such, we reasoned that our ETP leukemia model might be sensitive to bromodomain inhibitors such as JQ1. Indeed, we observed high sensitivity of expanded DKOITD ETPs to JQ1, raising the possibility of a new therapeutic approach for ETP leukemias.This novel mouse model of ETP-propagated leukemia, driven by clinically relevant mutations, provides intriguing evidence that leukemias with a predominant myeloid phenotype, but co-expressing lymphoid genes, may initiate within a bona fide early lymphoid progenitor population. Since the functional characteristics of the cell of origin of a leukaemia may direct its progression and response to therapy, these findings could have important implications for future stratification and treatment of both AML and ETP leukemias. DisclosuresNo relevant conflicts of interest to declare.

Dysregulation of Norepinephrine Release in the Absence of Functional Synaptotagmin 7.

Synaptotagmin 7 (Syt7) is expressed in cardiac sympathetic nerve terminals where norepinephrine (NE) is released in both Ca(2+)-dependent exocytosis and Ca(2+)-independent norepinephrine transporter (NET)-mediated overflow. The role of Syt7 in the regulation of NE release from cardiac sympathetic nerve terminals is tested by employing a Syt7 knock-in mouse line that expresses a non-functional mutant form of Syt7. In cardiac sympathetic nerve terminals prepared from these Syt7 knock-in mice, the Ca(2+)-dependent component of NE release was diminished. However, these terminals displayed upregulated function of NET (∼130% of controls) and a significant increase in Ca(2+)-independent NE overflow (∼140% of controls), which is greater than the Ca(2+)-dependent component of NE exocytosis occurring in wild-type controls. Consistent with a significant increase in NE overflow, the Syt7 knock-in mice showed significantly higher blood pressures compared to those of littermate wild-type and heterozygous mice. Our results indicate that the lack of functional Syt7 dysregulates NE release from cardiac sympathetic nerve terminals.